By your use of the term
Duo you are probably referring to the Intel line, not an IBM G5. The Core Duo chips are used in the mini, the iMac and the powerbook line. The desktops/servers use a woodcrest chip.
They are supposedly working very well. I haven't got any yet, I'm still on G3s, 4s, and 5s.
You can probably get something from the reviews at
http://www.macworld.com/Depending on what you're doing, the G5s are probably still very nice computers (and can be had for good prices) for most applications, and they will be supported for quite a while still. Heck some of my (original form) iMacs (G3s) still run the most current OS.
The one nice advantage to an IntelMac is that if you want to you can run Windows on it. So your machine can run OS X, BSD, and Windows.